Page 953 - Basic Electrical Engineering

antimony atom will make four covalent bonds with four germanium atoms.
By sharing of electrons in the covalent bonds all the atoms will satisfy their
need to have all the eight positions filled in their outermost orbit, i.e., their

valence shells.
It may be noted that the n-type semiconductor thus formed remains

electrically neutral, i.e., neither positively charged nor negatively charged.
This is because the total number of electrons including the free electrons is

equal to the total number of protons in the nuclei of the atoms.
The added impurity material has infact donated one free electron per atom

to the extrinsic semiconductor, and hence are called donor atoms. Donor
atoms create free electrons which form the majority charge carrier
(responsible for current flow) in an n-type material.

Temperature rise above absolute zero also creates free electrons and holes
due to breaking of covalent bonds, thus increasing the total number of free

electrons. However, a certain amount of holes are also formed.
When electrons leave their positions creating holes, the movement of

electrons gets associated with the movement of holes. The holes therefore
form charge carriers, and since they are in minority, they are called minority

charge carriers in the n-type semiconductor.
Thus, in an n-type semiconductor the majority charge carriers are the
electrons and the minority charge carriers are thermally generated holes.

14.4.2 P-Type Semiconductor Material

P-Type material is formed when silicon or germanium crystal is doped with
(added with) a small percentage of trivalent impurity material like boron,

gallium or indium. When covalent bonds are formed between boron having
three valence electrons with silicon having four valence electrons, there will

be shortage of one electron in the covalent bonds. This is represented by an
empty space in the covalent bonds and is called a hole as shown in Fig. 14.4.

There will be one hole corresponding to each of the impurity atoms taking
part in forming covalent bonds. This makes seven out of eight positions
filled.